(1) Field of the Invention
This invention relates to a novel silver halide emulsion which is able to produce highly sensitive silver halide photosensitive materials. More specifically, this invention relates to a novel emulsion containing halogen conversion-type silver halide fine grains.
(2) Description of the Prior Art
Halogen conversion-type emulsions have been known and prepared by, in the preparation of silver halide photographic emulsion, adding a halide enabling production of a silver halide having a smaller solubility product than that of hardly water soluble silver salt prepared in an aqueous solution to the aqueous solution, and thereby converting the halogen thereof into silver halide grains.
It is also well known that such halogen conversion-type emulsions are used as negative emulsions, positive emulsions, etc. and that the emulsions forms an internal latent image upon exposure. The emulsion enables development of a negative image upon treatment with an internal developing solution containing sodium thiosulfate and the like, followed by an imagewise exposure. Furthermore, it is well known to those skilled in the art that the emulsion directly forms a positive photosensitive material when treated with a developing solution in the presence of a fogging agent after imagewise exposure.
Halogen conversion-type emulsions are described in the specifications of Japanese Patent Pre-examined Publication (KOKAI) No. 18309/1977, KOKAI No. 30122/1981, Japanese Patent Publication No. 6090/1982 and etc. However, these specifications disclose processes for preparing a halogen conversion-type emulsion having good light response photographic properties such as sensitivity, gradation and maximum density, where the emulsion is treated with a general developer or specially treated by fogging development.
For example, Japanese KOKAI No. 18309/1977 discloses a halogen conversion-type emulsion which is prepared by conducting a precipitation reaction under the condition that hardly water soluble silver salts having a large solubility product are converted into hardly water soluble silver salts having a small solubility product, so that it provides an improved relationship between sensitivity and granularity of the emulsion as well as maximum density.
Japanese KOKAI No. 30122/1981 discloses a halide conversion-type emulsion containing grains having an average grain size of not more than 0.4.mu., which is prepared by conducting halogen conversion reaction at high temperature (for example 70.degree. C.) between silver chloride emulsion or silver chlorobromide emulsion and bromide or a mixture of bromide and iodide, after which chemical sensitization is carried out under special conditions, so that it provides an improvement in the sensitivity of the emulsion.
This Japanese Patent Publication also discloses that, bromide, iodide or a mixture thereof is added thereto within 20 minutes after formation of silver halide grains containing no silver iodine, thereby controlling physical ripening while controlling the growth of silver halide grains in order to prepare a high sensitive emulsion containing fine grains.
Furthermore, halogen conversion-type emulsions have been known from publications other than the above patents. For example, U.S. Pat. No. 2,592,250 discloses a halogen conversion-type silver chloroiodobromide emulsion containing at least 6 mol% of silver iodide in which silver chloride is converted by bromide and iodide. This patent can provide a silver halide emulsion enabling formation of a latent image at the interior.
On the other hand, as a rule, a silver halide containing silver iodide has been used in highly sensitive silver halide photographic materials. However, when the development is carried out in the presence of iodide such that the photographic material per se contains iodide as described above or a developing solution contains iodide, problems arise in that many changes occur in respect of photographic characters and image quality.
The above problems may, for example, result from the removal of iodide ion between silver halide grains in the same emulsion layer or other layers, or the irrigation of iodide ions from the developer to silver halide grains in the coating layer of the photographic layer.
Accordingly, there have been desired techniques for effectively trapping iodide ions. Such an effect cannot be adequately achieved by the halogen conversion-type emulsion described in the above patent specifications.